1. Field of the Invention
The present invention relates to AC power distribution systems and, more particularly, to soft switched AC power distribution systems.
2. Prior Art
AC distribution systems based on other conventional and soft switched topologies using flyback converters are currently in use. The purpose of such systems is to convert power from a DC source to AC, and distribute that power to one or more loads. In most applications, the power will be converted back to DC at the point of load. AC power distribution allows many different DC power outputs, referenced to many different circuit commons, to be created with a minimum number of circuit components. The AC link will radiate and couple electromagnetic interference (EMI) into other nearby equipment, which may degrade the performance of that equipment. The potential for EMI problems is exacerbated if the AC power waveforms contain high frequency harmonics. For this reason, it is advantageous to limit the risetimes of the voltage and current waveforms of the AC power. One method of limiting the risetimes without introducing additional loss is the use of a resonant circuit. This is commonly known as xe2x80x9csoft switchingxe2x80x9d. Several methods for accomplishing this have been successfully implemented, including one that uses a push-pull topology. The push-pull topology, and other bilateral circuits, requires less filtering at the input and outputs than a flyback, but requires more parts per DC output than a flyback, or other unipolar converter topology. In addition, flyback converters in several forms are in common use for DC to DC power conversions.
In general, flyback converters may be classified as:
a) SOPS (Self Oscillating Power Supply) converters in which the demagnetization of the transformer on an auxiliary winding is detected (verification of a complete transfer to the secondary circuit of the energy accumulated while driving the switch connected to the primary winding, during a subsequent off phase of the switch evidenced by the current through the secondary winding becoming null so that the successive turning on of the power switch takes place with a null current in the primary winding), to control the turning on of the power transistor. As an alternative, a null voltage across the switch may be detected before restoring current in the switch and primary winding, which provides xe2x80x9csoftxe2x80x9d turn-on of the switch. In this way, a xe2x80x9cdiscontinuousxe2x80x9d functioning mode is realized which is different from a continuous functioning mode in which the power switch is turned on with current still flowing in the secondary winding. This mode of operation is a discontinuous functioning mode that results in altering the switching frequency as a function of the power absorbed by the load connected to the converter output (secondary circuit of the transformer) and also as a function of the voltage of the input power source.
b) Fixed frequency converters operating in a discontinuous mode under nominal operating conditions. However, under other conditions, such as for example, during start-up conditions and when experiencing or recovering from short circuit events, they work in a continuous manner, unless the monitoring of the demagnetization is effected, typically on an auxiliary winding, for disabling the functioning of the oscillator that establishes the fixed switching frequency.
c) Fixed frequency converters operating in a continuous mode under normal operating conditions. In this operating mode, the primary switch is turned on while current is still flowing in the secondary winding. This operating mode reduces the RMS currents in the transformer windings and filter capacitors, but generally requires a larger transformer than is needed in a discontinuous converter.
As noted above, flyback converter configurations include circuits on the secondary windings for monitoring the output voltage and providing feedback to the primary side; where the feedback signal is used to control the switching oscillations of the power transistor. In this manner the voltage induced in the secondary winding may be regulated by controlling the time that current is allowed to flow in the primary winding before the current through the primary is turned off and the magnetizing field is allowed to collapse.
In addition, typical switching converters may also be characterized by the generation of undesirable electromagnetic interference (EMI). EMI may result from the high frequency and rapid rise-time of the induced current in the secondary winding and other voltage and current signals; where the rapid rise time is a result of the power transistor switching at an oscillation frequency of generally 20 Khz or greater.
A conventional solution for controlling EMI includes line filters in the power supply circuitry for rejecting the coupling of high frequency EMI signals. A common form of such a filter includes series connected inductances in the power supply branches, and shunt connected capacitors disposed either between power supply lines or between a power supply line and ground. Consideration of xe2x80x9cgroundxe2x80x9d or a xe2x80x9cpoint of common reference potentialxe2x80x9d as a part of the circuit is particularly important in rejecting common mode EMI, i.e. interference that is generated by the offending circuitry with reference to ground. The shunt line filter capacitance between the power supply and ground are helpful in alleviating EMI. However, adding inductors and capacitors to reduce EMI has the associated disadvantage of increasing manufacturing complexity and cost as well as increasing the necessary space required to accommodate the extra components.
These approaches are effective in limiting EMI in the DC input and output voltages, but have little or no effect on EMI radiated or coupled from the transformer, switching devices, and associated conductors. These EMI effects are especially prevalent in a distributed AC power system, in which AC signals are conducted over relatively long distances. Controlling the rise times and fall times of the voltages and currents can greatly reduce such EMI.
In accordance with one embodiment of the invention, an improved soft switched AC power distribution system for minimizing electromagnetic interference (EMI) and power loss is provided. The improved AC power distribution system comprises a flyback transformer comprising a primary winding and an intermediate secondary winding. A synchronization control circuit including a switch connectable to the primary winding controls the current through the primary winding. At least one load transformer, having a primary winding and at least one secondary winding, is connected to the intermediate secondary winding through an EMI shielded AC conduit. At least one output circuit (AC to DC converter) comprising a secondary winding coupled to the at least one load transformer, at least one rectifier connected to the at least one winding, and at least one output capacitor connected to the at least one rectifier is provided. The synchronization control circuit initiates current flow in the primary winding when the natural resonance of the circuit causes the voltage across the primary switch to collapse to zero. A resonating capacitance connectable to the primary switch controls voltage and current risetimes and minimizes power loss.
In accordance with another embodiment, the invention includes a method for providing an improved soft switched AC power distribution system. The method includes the steps of providing a flyback transformer having a primary winding and at least one secondary intermediate winding. The method further comprises the steps of providing current flow through the primary winding to induce energy storage in the flyback transformer and sinking the current flow through the primary winding in order to collapse the flyback transformer""s magnetic thereby transferring energy to the at least one secondary intermediate winding. The method further recites steps of providing a resonating capacitance connectable across the primary switch, and steps for sensing substantially null voltage across the primary switch, indicating that the switch may be turned on with minimal energy loss and EMI, and indicating substantial energy transfer to the at least one secondary winding. Upon sensing substantially null voltage across the primary switch, current flow through the primary winding is restored.
In accordance with another embodiment of the invention, a method for controlling electromagnetic interference (EMI) in a soft switched AC power distribution system is provided. The method comprises the steps of providing a flyback transformer, wherein the flyback transformer comprises a primary winding and at least one secondary intermediate secondary winding. The method further recites the steps of providing a synchronization control circuit for controlling current flow through the primary winding, and providing a resonating capacitance whose value is chosen to indirectly control voltage and current risetimes in the AC power distribution system, wherein voltage and current risetimes are associated with EMI.